Variants are not supported by parsers that lazily synthesize attributes e.g. `string`

[Kojoley]

If parser lazily synthesize attribute (e.g. string) it actually exposes a pair of iterators that will be used to build an attribute inplace by qi::assign_to/x3::move_to call.

But neither x3::move_to(first, last, attr) nor qi::assign_to(first, last, attr) support variants. So next code fails to compile while logically valid:

#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/home/x3.hpp>
#include <boost/variant.hpp>
#include <string>

int main()
{
    typedef boost::variant<int, std::string> attribute_type;
#if X3
    {
        namespace x3 = boost::spirit::x3;
        x3::rule<class r, attribute_type> r;
        auto start = r = x3::string("foo");

        // need to have a parse call to instantiate the templates
        attribute_type a;
        std::string s = "foo";
        x3::parse(s.cbegin(), s.cend(), start, a);
    }
#else
    {
        namespace qi = boost::spirit::qi;
        qi::rule<const char*, attribute_type()> r;
        r = qi::string("foo");
    }
#endif
    return 0;
}

[djowel]

Nods. Good catch!